To ensure smooth operation of engines, engine oils play an important role in lubricating a variety of sliding parts in the engine, for example, piston rings/cylinder liners, bearings of crankshafts and connecting rods, valve mechanisms including cams and valve lifters, and the like. Engine oils may also play a role in cooling the inside of an engine and dispersing combustion products. Further possible functions of engine oils may include preventing or reducing rust and corrosion.
The principal consideration for engine oils is to prevent wear and seizure of parts in the engine. Lubricated engine parts are mostly in a state of fluid lubrication, but valve systems and top and bottom dead centers of pistons are likely to be in a state of boundary and/or thin-film lubrication. The friction between these parts in the engine may cause significant energy losses and thereby reduce fuel efficiency.
In order to improve fuel efficiency, friction between engine parts must be reduced. Thin-film friction is the friction generated by a fluid, such as a lubricant, moving between two surfaces when the distance between the two surfaces is very small. It is known that some additives normally present in engine oils form films of different thicknesses which can have an effect on thin-film friction. Some additives, such as zinc dialkyl dithio phosphate (ZDDP), are known to increase thin-film friction. Though such additives may be required for other reasons, such as to protect engine parts, the increase in thin-film friction caused by such additives can be detrimental.
Both inorganic and organic friction modifiers have been utilized to help reduce thin-film friction and improve engine lubrication. For example:
U.S. Pat. Nos. 5,885,942, 5,866,520, 5,114,603, 4,957,651 and 4,683,069 and published applications WO 2011/107739, US 2012/0129743 and US 2014/0179571 disclose glycerol monooleate (GMO) and its use as a friction modifier for lubricant compositions;
WO2013/176725 discloses metal based friction modifiers, such as organo molybdenum friction modifiers, blended with short chain alkyl esters of hydroxy carboxylic acids and their use in lubricant compositions;
WO2012/162282 discloses friction modifiers having one or more amide functional groups;
WO2012/162027 discloses an amine salt of a hydrocarbyl phosphate, a hydrocarbyl thiophosphate, a hydrocarbyl dithiophosphate, or combinations thereof and its use as a friction modifier;
WO2012/071185 discloses a friction modifier composition containing: a) an amino alcohol reaction product prepared by isomerizing a C12-C30 normal alpha olefin using at least one of a solid or liquid catalyst to form an internal olefin; epoxidizing said olefin; and reacting with an mono- or di-hydroxyl hydrocarbyl amine and b) an ester of glycerol and a C12-C22 carboxylic acid containing 0 to 3 double bonds;
EP0884378 discloses chain-hydrocarbon-group-substituted metal salicylates having friction reducing ability;
EP0389273 discloses a long chain succinimide derivative and a long chain amide described to have superior friction reducing properties; and
US 2011/0028364 discloses alkenyl-substituted succinamide friction modifiers and their use in lubricant compositions to reduce friction.
Given the increasing fuel economy demands placed on engines, there remains a need to further improve the friction reduction and fuel economy of internal combustion engines utilizing lubricant compositions. It is therefore desirable to improve on the friction-reducing performance of known friction modifiers, such as glycerol monooleate, that have been commonly used in the art.